The invention relates to an electromechanical drive or a sensor element for a measurement instrument, and a method for their production. It relates in particular to drives and sensor elements manufactured in the form of stacks and composed of piezoelectric elements, and to measurement instruments equipped with such drives and sensor elements.
Known measurement instruments of this type include, for example, piezoelectric acceleration sensors and level measurement instruments. These measurement instruments generally comprise a base body on which the drive or the sensor element is fitted, with these items containing piezoelectric elements, electrodes and electrical connectors and connections. The piezoelectric elements are electrically connected to one another, and are connected to an electronics circuit appropriate to the application.
Such piezoelectric measurement instruments are relatively highly sensitive to electrostatic charges and have relatively low transverse sensitivity. However, their particular disadvantage is their relatively limited effective operating range at high temperatures above, for example, 200xc2x0 C., under the influence of high static pressures of up to, for example, 500 bar or, in the case of acceleration sensors, when subject to high dynamic loads, during which accelerations of up to 2000 g occur.
The sensitivity of the piezoelectric measurement instruments to the described loads is dependent on the design of their drive or sensor element. Normally, such a drive or such a sensor element is formed from piezoelectric elements which follow one another and are geometrically matched to one another, electrodes as well as electrical connectors and connections. Various materials, such as piezoceramic and metal, are thus combined with one another and their moduli of elasticity and their respective coefficients of thermal expansion differ. Extended mechanical planar stresses, which reduce not only the strength of the sensor element but also the wanted signal or measurement signal, are produced in the piezoelectric elements themselves under the influence of the said high temperatures and/or large static and dynamic loads on the sensor element or on the drive.
In known measurement instruments of the described type, a monolithic thin-film capacitor with a layer structure has also been found to be advantageous, which may also be in the form of a multiple capacitor composed of thin layers, for example for level measurement instruments. In this case, the layer sequence of the monolith comprises the piezoelectric thin films and electrodes, with electrical connectors being provided, arranged along the monolith. Normally, a connector (often also referred to as a xe2x80x9crailxe2x80x9d) then connects the electrodes of the one pole, and another connector connects the electrodes of the other pole. Such a monolithic structure is highly robust, has high strength and allows vibration parameters to be measured very accurately by reducing the relative coefficients of the transverse transformation.
However, this structure is based on an asymmetric electrode shape, with a projecting part for connection to a rail on one side and with an isolating clearance from the other rail on the opposite side. Inhomogeneities in the structural design of such drives or sensor elements have a negative effect on the magnitude of the relative transverse transformation coefficient, however. Furthermore, the asymmetric shape of the electrodes does not allow the entire electrode area of the piezoelectric layers to be charged up, as would be desirable in order to enlarge the transverse transformation coefficient. The smaller the metallization area of the piezoelectric layer under consideration in comparison with its actual useful area, the greater is the extent to which the transformation coefficient is reduced.
The invention is thus based on the object of specifying piezoelectric drives and sensor elements, and a method for their production which allow the disadvantages described above to be avoided with measurement instruments equipped in such a way, and which are also distinguished by high measurement accuracy under the influence of high temperatures, and high static and dynamic loads.
This object is achieved by a first variant of the invention by means of an electromechanical drive or a sensor element having a layer structure, which comprises
a plurality of piezoelectric ceramic layers,
an electrode layer which is arranged between two mutually facing surfaces of directly adjacent piezoelectric ceramic layers, and
an electrical connector for making electrical contact with the electrode layer,
in which case the connector is likewise arranged and is passed out between the two mutually facing surfaces of the piezoelectric ceramic layers.
This object is furthermore achieved by a second variant of the invention by means of an electromechanical drive or sensor element having a layer structure,
having a plurality of piezoelectric ceramic layers,
in which mutually facing surfaces of directly adjacent piezoelectric ceramic layers are metallized by application of a metal coating,
which are joined together by means of diffusion welding,
so that an electrode layer is formed by the metallized surfaces,
with which contact can be made via an electrical connector.
One preferred embodiment of the invention provides that a groove is provided in at least one of the two mutually facing surfaces of the piezoelectric ceramic layers and at least partially holds the electrical connector.
In another preferred embodiment of the invention, the connector is a wire which extends beyond the surfaces of the piezoelectric ceramic layers.
In yet another preferred embodiment of the drive or sensor element according to the invention, at least three piezoelectric ceramic layers and at least two grooves are provided, with these grooves being arranged offset with respect to one another and with respect to a longitudinal axis of the drive or sensor element.
In a development of a preferred embodiment of the invention, the wire has a rippled or zigzag structure.
In other developments of the invention, the piezoelectric ceramic layers are composed of PZT material, such as PbMg0.308Nb0.617Ti0.075O3, or they are formed from ceramic layers composed of a material having a Curie temperature of more than 400xc2x0 C., for example composed of Na0.5Bi4.5Ti4O15 or Bi3TiNbO9.
In other developments of the invention, the electrode layers are composed of a metallic material having a Curie temperature of more than 400xc2x0 C., such as bismuth-titanate.
Other developments, in addition, of the invention relate to wires composed of a metallic material having high-temperature stability at more than 250xc2x0 C. and wires composed of a material which contains silver and stainless steel, or a material which contains a nickel alloy.
The said object on which the invention is based is, furthermore, achieved by an electromechanical drive or a sensor element having a layer structure which is produced using a method which comprises the following steps:
production of ceramic layers composed of electrically active material using a method which is normal in ceramic technology, having desired dimensions and having a margin of 2-3 mm for each dimension taking account of the following mechanical machining;
grinding the ceramic layers until a predetermined thickness of, for example, 0.15 to 03 mm is reached;
cutting a groove in one face of the ceramic layers which is to be metallized;
in which case the depth of the groove must be no deeper than half the thickness of the ceramic layer under consideration;
coating at least one face of the ceramic layers with metal by applying a paste containing silver twice and subsequent heat treatment at a temperature of 800-820xc2x0 C.;
applying adhesive to the metallized surfaces of two ceramic layers using cellulose adhesive;
diffusion welding of the layers to which adhesive has been applied by heat treatment at a temperature of 780-800xc2x0 C. and single-axis compression at a pressure of 3-5 kg/cm2 over a period of 3 hours and cooling to room temperature;
drawing in each case one connector wire into a groove;
action of an electric field on the wires at high temperature and setting of the desired polarity of the electrode layers by connection of the same poles of the drive or of the sensor element;
checking of the desired parameters and piezoelectric characteristics of the drive or of the sensor element.
In addition, a level limit switch is provided which is equipped with a drive and a sensor element according to the invention, with the sensor element in a development of such a level limit switch being separated from the drive by a non-polarized ceramic layer.
Furthermore, an acceleration sensor is provided, having a sensor element according to the invention.
The particular advantage is that the construction according to the invention and the production method according to the invention make it possible to produce a piezoelectric drive or a sensor element in which the charge transformation coefficient and the measurement accuracy of the sensor element are improved in that the relative transverse transformation coefficient is reduced over a wide temperature range and during high static and dynamic loads.
The invention will be explained and described in detail in the following text using a number of exemplary embodiments and with reference to a drawing, in which: